Electrode material, method for manufacturing electrode material, electrode, and lithium ion battery

ABSTRACT

An electrode material including a carbonaceous-coated electrode active material having primary particles of an electrode active material, secondary particles that are aggregates of the primary particles, and a carbonaceous film that coats the primary particles of the electrode active material and the secondary particles that are the aggregates of the primary particles, in which, in the electrode material, when ten random 180 nm×180 nm views are observed using an electron microscope at a magnification of 100,000 times, the number of free carbon aggregates is three or less, and the number of protrusions twice or more as thick as the carbonaceous film is three or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No.2018-182511 filed Sep. 27, 2018, the disclosure of which is hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an electrode material, a method formanufacturing the electrode material, an electrode formed of theelectrode material, and a lithium ion battery including a cathode madeof the electrode.

Description of Related Art

In recent years, as small-size, lightweight, and high-capacitybatteries, non-aqueous electrolyte-based secondary batteries such aslithium ion batteries have been proposed and put into practical use.Lithium ion batteries are constituted of a cathode and an anode whichhave properties capable of reversibly intercalating and deintercalatinglithium ions and a non-aqueous electrolyte.

Lithium ion batteries weigh less and have a smaller size and a higherenergy than secondary batteries of the related art such as leadbatteries, nickel-cadmium rechargeable batteries, and nickel metalhydride rechargeable batteries, are used as power supplies for mobileelectronic devices such as mobile phones and notebook-type personalcomputers, and, in recent years, also have been studied as high-outputpower supplies for electric vehicles, hybrid vehicles, and electrictools. Electrode active materials for batteries that are used as theabove-described high-output power supplies are required to havehigh-speed charge and discharge characteristics. In addition, studiesare also made regarding the smoothing of power generation loads or theapplication to large-scale batteries such as stationary power suppliesand backup power supplies, and the absence of problems with resourceamounts as well as long-term safety and reliability is also consideredto be important.

Cathodes in lithium ion batteries are constituted of an electrodematerial including a lithium-containing metal oxide having propertiescapable of reversibly intercalating and deintercalating lithium ionswhich is called a cathode active material, a conductive auxiliary agent,and a binder, and this electrode material is applied onto the surface ofa metallic foil which is called a current collector, thereby producingcathodes. As the cathode active material for lithium ion batteries,generally, lithium cobalt oxide (LiCoO₂) is used, and, additionally,lithium (Li) compounds such as lithium nickel oxide (LiNiO₂), lithiummanganese oxide (LiMn₂O₄), and lithium iron phosphate (LiFePO₄) areused. Among these, lithium cobalt oxide or lithium nickel oxide has aproblem of the toxicity or resource amounts of elements and a problemsuch as instability in charged states. In addition, lithium manganeseoxide is pointed out to have a problem of being dissolved inelectrolytes at high temperatures. On the other hand, lithium ironphosphate has excellent long-term safety and reliability, and thusphosphate-based electrode materials having an olivine structure, whichare represented by lithium iron phosphate, have been attractingattention in recent years (for example, refer to Japanese Laid-openPatent Publication No. 2013-161654).

SUMMARY OF THE INVENTION

The phosphate-based electrode materials described in Japanese Laid-openPatent Publication No. 2013-161654 have insufficient electronconductivity and thus, in order to charge and discharge large electriccurrents, a variety of means such as the miniaturization of particlesand the conjugation with conductive substances is required, and a lot ofefforts are being made.

However, conjugation using a large amount of a conductive substancecauses a decrease in electrode densities, and thus a decrease in thedensity of batteries, that is, a decrease in capacities per unit volumeis caused. As a method for solving this problem, a carbon coating methodusing an organic substance solution as a carbon precursor which is anelectron conductive substance has been found. In this method in whichthe organic substance solution and electrode active material particlesare mixed together and then the mixture is dried and thermally treatedin a non-oxidative atmosphere, thereby carbonizing an organic substance,it is possible to extremely efficiently coat the surfaces of theelectrode active material particles with a minimum necessary amount ofthe electron conductive substance, and conductivity can be improvedwithout significantly decreasing electrode densities.

However, the carbonization temperature of the organic substance which isa carbon source is generally a high temperature, and thus, during themanufacturing of these electrode materials, electrode active materialparticles come into contact with one another, some of the electrodeactive material particles are sintered during high-temperaturecarbonization, the particles grow, and it is not easy to produce fineparticles.

In addition, when the amount of the organic substance is increased inorder to ensure sufficient conductivity, a carbide of the organicsubstance which does not contribute to coating or is more than necessaryfor the supply of electrons to the surfaces of electrode active materialparticles remains. There is a concern that this poorly carbonaceouscarbide may be decomposed during cycle charging and discharging and thusthis carbonaceous carbide may cause an increase in resistance and adecrease in capacity.

Generally, electrode materials for lithium ion batteries are used afterbeing turned into paste by adding a material referred to as a conductiveauxiliary agent such as carbon black, a binder represented bypolyvinylidene fluoride, and a solvent and then applied onto a metallicfoil referred to as a current collector. In order to use a fineelectrode material in the above-described paste form, there has been aproblem in that a large amount of the solvent is used in order to adjustthe viscosity to be suitable for application and a large amount of thebinder is used in order to ensure the adhesive force to the currentcollector, which increases the amounts of materials other than theelectrode material used and time and efforts are required to remove thesolvent.

The present invention has been made in consideration of theabove-described circumstances, and an object of the present invention isto provide an electrode material enabling the obtainment of lithium ionbatteries which are excellent in terms of the discharge capacity and theinput and output characteristics, suppress an increase in resistance incycle charging and discharging, and also have excellent cyclecharacteristics, a method for manufacturing the electrode material, anelectrode formed of the electrode material, and a lithium ion batteryincluding a cathode made of the electrode.

The present inventors carried out intensive studies in order to solvethe above-described problem and consequently found that, when an ionicorganic substance is used as a carbon source, electrode active materialparticles coming close to one another are suppressed, the particlegrowth and sintering of an electrode active material due to hightemperatures do not easily occur, and fine electrode active materialparticles coated with a favorable carbonaceous film are obtained. Inaddition, it was found that an organic substance is adsorbed to thesurfaces of the electrode active material particles with no waste, theorganic substance is not freed from the surfaces of the electrode activematerial particles even during drying and granulation and duringhigh-temperature carbonization, it becomes possible to appropriatelycoat the surfaces of the electrode active material particles, and thusthe above-described problem can be solved.

The present invention has been completed on the basis of theabove-described findings.

That is, the present invention provides the following [1] to [6].

[1] An electrode material including a carbonaceous-coated electrodeactive material having primary particles of an electrode activematerial, secondary particles that are aggregates of the primaryparticles, and a carbonaceous film that coats the primary particles ofthe electrode active material and the secondary particles that are theaggregates of the primary particles, in which, in the electrodematerial, when ten random 180 nm×180 nm views are observed using anelectron microscope at a magnification of 100,000 times, the number offree carbon aggregates is three or less, and the number of protrusionstwice or more as thick as the carbonaceous film is three or less.

[2] The electrode material according to [1], in which the electrodeactive material is an electrode active material represented by GeneralFormula (1).

Li_(a)A_(x)M_(y)BO_(z)  (1)

(In the formula, A represents at least one element selected from thegroup consisting of Mn, Fe, Co, and Ni, M represents at least oneelement selected from the group consisting of Na, K, Mg, Ca, Al, Ga, Ti,V, Cr, Cu, Zn, Y, Zr, Nb, Mo, and rare earth elements, B represents atleast one element selected from the group consisting of B, P, Si, and S,0≤a<4, 0<x<1.5, 0≤y<1, and 0<z≤4.)

[3] The electrode material according to [2], in which the electrodeactive material represented by General Formula (1) is an electrodeactive material represented by General Formula (2).

Li_(a)A_(x)M_(y)PO₄  (2)

(In the formula, A, M, a, x, and y are as described above.)

[4] A method for manufacturing the electrode material according to anyone of [1] to [3], including: a first step of drying and granulating aslurry obtained by mixing an ionic organic substance as a carbon source,one or more selected from an electrode active material and an electrodeactive material precursor, and a solvent using a spray dryer and asecond step of thermally treating the granulated substance obtained inthe first step in a non-oxidative atmosphere at 600° C. or higher and1,000° C. or lower.

[5] An electrode formed of the electrode material according to any oneof [1] to [3].

[6] A lithium ion battery including: a cathode made of the electrodeaccording to [5].

According to the present invention, it is possible to provide anelectrode material enabling the obtainment of lithium ion batterieswhich are excellent in terms of the discharge capacity and the input andoutput characteristics, suppress an increase in resistance in cyclecharging and discharging, and also have excellent cycle characteristics,a method for manufacturing the electrode material, an electrode formedof the electrode material, and a lithium ion battery including a cathodemade of the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an image of a cross section obtained by capturing an electrodematerial of Example 3 using a transmission electron microscope (TEM) (ata magnification of 100,000 times).

FIG. 2 is an image of a cross section obtained by capturing an electrodematerial of Comparative Example 2 using a transmission electronmicroscope (TEM) (at a magnification of 100,000 times).

DETAILED DESCRIPTION OF THE INVENTION

Electrode Material

An electrode material of the present embodiment is an electrode materialincluding a carbonaceous-coated electrode active material having primaryparticles of an electrode active material, secondary particles that areaggregates of the primary particles, and a carbonaceous film that coatsthe primary particles of the electrode active material and the secondaryparticles that are the aggregates of the primary particles,

in which, in the electrode material, when ten random 180 nm×180 nm viewsare observed using an electron microscope at a magnification of 100,000times, the number of free carbon aggregates is three or less, and thenumber of protrusions twice or more as thick as the carbonaceous film isthree or less.

The electrode active material that is used in the present embodiment isconstituted of primary particles and secondary particles that areaggregates of the primary particles. The shape of the electrode activematerial particle is not particularly limited, but is preferablyspherical, particularly, truly spherical. When the electrode activematerial particle has a spherical shape, filling properties ofelectrodes improve, and it becomes easier to obtain electrodes having ahigher density. In addition, when the carbonaceous-coated electrodeactive material is produced in a granular body form, it is possible todecrease the amount of a solvent used to prepare a paste for forming anelectrode using the electrode material of the present embodiment, and italso becomes easy to apply the paste for forming an electrode to acurrent collector. Meanwhile, the paste for forming an electrode can beprepared by, for example, mixing the electrode material of the presentembodiment, a binder resin (binder), and a solvent.

The electrode active material that is used in the electrode material ofthe present embodiment is preferably an electrode active materialrepresented by General Formula (1) from the viewpoint of a highdischarge capacity, a high energy density, safety, and cycle stability.

Li_(a)A_(x)M_(y)BO_(z)  (1)

(In the formula, A represents at least one element selected from thegroup consisting of Mn, Fe, Co, and Ni, M represents at least oneelement selected from the group consisting of Na, K, Mg, Ca, Al, Ga, Ti,V, Cr, Cu, Zn, Y, Zr, Nb, Mo, and rare earth elements, B represents atleast one element selected from the group consisting of B, P, Si, and S,0≤a<4, 0<x<1.5, 0≤y<1, and 0<z≤4.)

In the formula, A is at least one element selected from the groupconsisting of Mn, Fe, Co, and Ni, and, among these, Mn and Fe arepreferred, and Fe is more preferred.

M is at least one element selected from the group consisting of Na, K,Mg, Ca, Al, Ga, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, and rare earthelements, and, among these, Mg, Ca, Al, and Ti are preferred.

B is at least one element selected from the group consisting of B, P,Si, and S, and, among these, P is preferred from the viewpoint ofexcellent safety and cycle characteristics.

a is 0 or more and less than 4, preferably 0.5 or more and 3 or less,more preferably 0.5 or more and 2 or less, and particularlypreferably 1. x is more than 0 and less than 1.5, preferably 0.5 or moreand 1 or less, and, among these, 1 is preferred. y is 0 or more and lessthan 1 and preferably 0 or more and 0.1 or less. z is more than 0 and 4or less and is selected depending on the composition of B. For example,in a case in which B is phosphorus (P), z is preferably 4, and, in acase in which B is boron (B), z is preferably 3.

The electrode active material represented by General Formula (1)preferably has an olivine structure, is more preferably an electrodeactive material represented by General Formula (2), and still morepreferably LiFePO₄ or Li (Fe_(x1)Mn_(1-x1)) PO₄ (here 0<x1<1) which isLiFePO₄ in which some of Fe is substituted by Mn.

Li_(a)A_(x)M_(y)PO₄  (2)

(In the formula, A, M, a, x, and y are as described above.)

As the electrode active material (Li_(a)A_(x)M_(y)BO_(z)) represented byGeneral Formula (1), an electrode active material manufactured using amethod of the related art such as a solid-phase method, a liquid-phasemethod, or a gas-phase method can be used.

Li_(a)A_(x)M_(y)BO_(z) is obtained by, for example, hydrothermallysynthesizing a slurry-form mixture obtained by mixing a Li source, an Asource, an M source, a B source, and water and cleaning the obtainedprecipitate with water. In addition, the same electrode active materialis obtained by generating an electrode active material precursor bymeans of a hydrothermal synthesis and, furthermore, calcinating theelectrode active material precursor. A pressure-resistant airtightcontainer is preferably used in the hydrothermal synthesis.

Here, examples of the Li source include lithium salts such aslithiumacetate (LiCH₃COO) andlithiumchloride (LiCl), lithium hydroxide(LiOH), and the like, and at least one selected from the groupconsisting of lithium acetate, lithium chloride, and lithium hydroxideis preferably used.

Examples of the A source include chlorides, carboxylates, hydrosulfates,and the like which include at least one element selected from the groupconsisting of Mn, Fe, Co, and Ni. For example, in a case in which the Asource is Fe, examples of the Fe source include divalent iron salts suchas iron (II) chloride (FeCl₂), iron (II) acetate (Fe(CH₃COO)₂), and iron(II) sulfate (FeSO₄), and, among these, at least one selected from thegroup consisting of iron (II) chloride, iron (II) acetate, and iron (II)sulfate is preferably used.

As the M source, similarly, it is possible to use chlorides,carboxylates, hydrosulfates, and the like of Na, K, Mg, Ca, Al, Ga, Ti,V, Cr, Cu, Zn, Y, Zr, Nb, Mo, and rare earth elements.

Examples of the B source include compounds including at least oneelement selected from the group consisting of B, P, Si, and S. Forexample, in a case in which the B source is P, examples of the P sourceinclude phosphoric acid compounds such as phosphoric acid (H₃PO₄),ammonium dihydrogen phosphate (NH₄H₂PO₄), diammonium phosphate((NH₄)₂HPO₄), and the like, and at least one selected from the groupconsisting of phosphoric acid, ammonium dihydrogen phosphate, anddiammonium phosphate is preferably used.

The substance amount ratio (Li:A:M:B) of the Li source, the A source,the M source, and the B source is appropriately selected so that adesired electrode active material is obtained and impurities are notgenerated.

The crystallite diameter of the electrode active material is preferably30 nm or more and 250 nm or less, more preferably 35 nm or more and 250nm or less, and still more preferably 40 nm or more and 200 nm or less.When the crystallite diameter is 30 nm or more, the amount of carbonnecessary to sufficiently coat the electrode active material surfacewith a carbonaceous film is suppressed, and the amount of the binder canbe suppressed, and thus it is possible to increase the amount of theelectrode active material in electrodes and increase the capacities ofbatteries. Similarly, it is possible to prevent the easy occurrence offilm peeling caused by the lack of the binding force is also likely tobe caused. On the other hand, when the crystallite diameter is 250 nm orless, the internal resistance of the electrode active material issuppressed, and, in a case in which batteries are formed, it is possibleto increase the discharge capacities at a high charge-discharge rate.

Meanwhile, the crystallite diameter can be calculated from theScherrer's equation using the full width at half maximum of thediffraction peak and the diffraction angle (2θ) of the (020) plane in apowder X-ray diffraction pattern that is measured and obtained using anX-ray diffractometer (for example, RINT2000, manufactured by RigakuCorporation).

The carbonaceous film that coats the primary particles of the electrodeactive material and the secondary particles that are the aggregates ofthe primary particles is obtained by carbonizing an organic substancemade of a raw material of the carbonaceous film; however, in order todevelop a desired effect, an organic substance (ionic organic substance)having ionicity which has an excellent adsorption capability to thesurfaces of the electrode active material particles and is capable ofsuppressing the electrode active material particles coming close to oneanother through charge repulsion and a steric barrier is used. The ionicorganic substance is not particularly limited as long as the ionicorganic substance is capable of forming the carbonaceous film on thesurfaces of the electrode active material particles, and examplesthereof include alkyl benzene sulfonate, alkyl sulfonate, salts ofcarboxylic acid-modified polyvinyl alcohols, salts of sulfonicacid-modified polyvinyl alcohols, polycarboxylates, polyacrylates,polymethacrylates, and the like. In addition, commercially availableionic surfactants can also be preferably used.

In addition, the kind of a counter ion that the ionic organic substancehas is not particularly limited; however, in order to avoid theincorporation of unnecessary metal impurities, an ammonium ion, analkylammonium ion, a pyridinium ion, or the like is preferred. Inaddition, for the purpose of adjusting the composition, it is alsopossible to use a Li ion.

For the same reason, it is also possible to preferably use a cationicorganic substance represented by a polyamine salt such as an anilinesalt, a polyaniline salt, or the like. In addition, as the counter ionwhen the ionic organic substance is used as a carbon source, an aceticacid ion, a nitric acid ion, a phosphoric acid ion, or the like ispreferred.

The ionization ratio of the ionic organic substance needs to beappropriately adjusted in order to obtain a desired adsorptioncapability. For example, it is possible to neutralize an acidic organicsubstance such as polycarboxylate with an alkaline substance. Theneutralization ratio is preferably 30 mol % or more. The upper limitvalue is not particularly limited.

Neutralization can be carried out using an alkali such as ammonia, anorganic amine, or an alkali metal hydroxide. Among these, ammonia ispreferred since there is no concern of the remaining of unnecessarymetal.

Similarly, it is also possible to neutralize a basic organic substancesuch as polyaniline with an acidic substance such as acetic acid.

The ionic organic substance to be used is preferably soluble in solventsfor the reason of the easiness of mixing the ionic organic substance andthe electrode active material particles and for the purpose of obtaininga uniform coating of the carbonaceous film, and the ionic organicsubstance is more preferably soluble in water from the viewpoint ofdisassociation into ions, easiness of handling, safety, prices, and thelike.

These ionic organic substances may be used singly or two or more ionicorganic substances may be used in mixture.

The average particle diameter of the primary particles of the electrodeactive material coated with the carbonaceous film (carbonaceous-coatedelectrode active material) is preferably 30 nm or more and 250 nm orless, more preferably 50 nm or more and 200 nm or less, still morepreferably 50 nm or more and 150 nm or less, and far still morepreferably 60 nm or more and 100 nm or less. When the average particlediameter is 30 nm or more, it is possible to decrease the amount of thebinder necessary for the production of electrodes and increase thecapacities of batteries by increasing the amount of the electrode activematerial in electrodes. In addition, it is possible to suppress filmpeeling caused by the lack of the binding force. On the other hand, whenthe average particle diameter is 250 nm or less, it is possible toobtain sufficient high-speed charge and discharge performance.

Here, the average particle diameter of the primary particles refers tothe number-average particle diameter. The average particle diameter ofthe primary particles can be obtained by number-averaging the particlediameters of 200 or more particles measured by scanning electronmicroscope (SEM) observation.

The average particle diameter of the secondary particles of thecarbonaceous-coated electrode active material is preferably 0.5 μm ormore and 200 μm or less, more preferably 1 μm or more and 150 μm orless, and still more preferably 3 μm or more and 100 μm or less. Whenthe average particle diameter of the secondary particles is 0.5 μm ormore, it is possible to suppress an increase in the amount of theconductive auxiliary agent and the binder necessary to prepare electrodematerial paste by mixing the electrode material, the conductiveauxiliary agent, the binder resin (binder), and the solvent. Therefore,it is possible to increase the battery capacities of lithium ionbatteries. On the other hand, when the average particle diameter is 200μm or less, it is possible to increase the discharge capacities oflithium ion batteries in high-speed charge and discharge.

Here, the average particle diameter of the secondary particles refers tothe volume-average particle diameter. The average particle diameter ofthe secondary particles can be measured using a laser diffraction andscattering-type particle size distribution analyzer or the like.

The thickness (average value) of the carbonaceous film that coats theelectrode active material particles is preferably 0.5 nm or more and 6nm or less, more preferably 0.8 nm or more and 5 nm or less, and stillmore preferably 0.8 nm or more and 3 nm or less. When the thickness ofthe carbonaceous film is 0.5 nm or more, it is possible to suppress anincrease in the total of the migration resistances of electrons in thecarbonaceous film. Therefore, it is possible to suppress an increase inthe internal resistance of lithium ion batteries and prevent voltagedrop at a high charge-discharge rate. On the other hand, when thethickness is 6 nm or less, it is possible to suppress the formation of asteric barrier that hinders the diffusion of lithium ions in thecarbonaceous film, and thus the migration resistance of lithium ionsdecreases. As a result, an increase in the internal resistance ofbatteries is suppressed, and it is possible to prevent voltage drop at ahigh charge-discharge rate.

Meanwhile, the thickness of the carbonaceous film can be obtained bycapturing the carbonaceous-coated electrode active material using atransmission electron microscope (TEM) or a scanning electron microscope(SEM), measuring the thickness of the carbonaceous film at 100 placesfrom the obtained image of the cross section, and calculating theaverage value.

The coating ratio of the carbonaceous film to the electrode activematerial particles is preferably 60% or more and more preferably 80% ormore. When the coating ratio of the carbonaceous film is 60% or more,the coating effect of the carbonaceous film can be sufficientlyobtained.

Meanwhile, the coating ratio of the carbonaceous film can be obtained byobserving the carbonaceous-coated electrode active material using atransmission electron microscope (TEM), an energy dispersive X-raymicroanalyzer (EDX), or the like, calculating the proportions of acovered portion in the electrode active material surface, andcalculating the average value.

The density of the carbonaceous film is preferably 0.2 g/cm³ or more and2 g/cm³ or less and more preferably 0.5 g/cm³ or more and 1.5 g/cm³ orless. The density of the carbonaceous film refers to the mass per unitvolume of the carbonaceous film.

When the density of the carbonaceous film is 0.2 g/cm³ or more, thecarbonaceous film exhibits sufficient electron conductivity. On theother hand, when the density of the carbonaceous film is 2 g/cm³ orless, the amount of the crystals of graphite having a lamellar structurein the carbonaceous film is small, and thus the steric barrier by thefine crystals of graphite is not caused when lithium ions diffuse in thecarbonaceous film. Therefore, there is no case in which the lithium ionmigration resistance increases. As a result, there is no case in whichthe internal resistance of lithium ion batteries increases, and voltagedrop at a high charge-discharge rate of lithium ion batteries is notcaused.

The content of carbon included in the electrode material of the presentembodiment is preferably 0.5% by mass or more and 3.5% by mass or less,more preferably 0.8% by mass or more and 2.5% by mass or less, and stillmore preferably 0.8% by mass or more and 2.0% by mass or less. When thecontent of carbon is 0.5% by mass or more, it is impossible tosufficiently increase electron conductivity. On the other hand, when thecontent of carbon is 3.5% by mass or less, it is possible to increaseelectrode densities.

Meanwhile, the content of carbon can be measured using a carbon analyzer(for example, manufactured by Horiba Ltd., carbon/sulfur analyzer:EMIA-810W).

The specific surface area of the electrode material of the presentembodiment is preferably 10 m²/g or more and 30 m²/g or less, morepreferably 12 m²/g or more and 28 m²/g or less, and still morepreferably 15 m²/g or more and 27 m²/g or less. When the specificsurface area is 10 m²/g or more, it is possible to develop sufficienthigh-speed charge and discharge performance. On the other hand, when thespecific surface area is 30 m²/g or less, it is possible to constituteelectrodes without including a large amount of the binder and theconductive auxiliary agent, and thus it is possible to suppress adecrease in the capacities of batteries.

Meanwhile, the specific surface areas can be measured using a specificsurface area meter (for example, manufactured by Mountech Co., Ltd.,trade name: Macsorb HM model-1208) and the BET 1 point method by meansof nitrogen (N₂) adsorption.

In the electrode material of the present embodiment, when ten random 180nm×180 nm views are observed using an electron microscope at amagnification of 100,000 times, the number of free carbon aggregates isthree or less. When the number of free carbon aggregates is more thanthree, the poorly carbonaceous carbide which does not contribute to thecoating of the electrode active material particles increases, and, in acase in which batteries have been formed, the carbide is decomposedduring cycle charging and discharging, and there is a concern that anincrease in resistance and a decrease in capacity may be caused. Fromthe above-described viewpoint, the number of free carbon aggregates ispreferably two or less, more preferably one or less, and particularlypreferably zero.

As the electron microscope, it is possible to use a transmissionelectron microscope (TEM) or a scanning electron microscope (SEM). Anaccelerated voltage is preferably 10 to 200 kV.

Meanwhile, the free carbon aggregates refers to an irregularcarbonaceous lump freed from the carbonaceous-coated electrode activematerial. The number of free carbon aggregates can be obtained byobserving the electrode material using the electron microscope under theabove-described conditions and counting the number of irregularcarbonaceous lumps freed from the carbonaceous-coated electrode activematerial. Meanwhile, free carbon aggregates are observed to be deformedduring the radiation of electron beams by an accelerated voltage of 200kV and thus can be easily differentiated from carbon not freed from thecarbonaceous film.

In addition, in the electrode material of the present embodiment, whenten random 180 nm×180 nm views are observed using the electronmicroscope at a magnification of 100,000 times, the number ofprotrusions twice or more as thick as the carbonaceous film is three orless. When the number of protrusions twice or more as thick as thecarbonaceous film is more than three, a carbide (excess carbide) morethan necessary for the supply of electrons to the electrode activematerial particle surfaces is present, and, in a case in which batterieshave been formed, the excess carbide is decomposed during cycle chargingand discharging, and there is a concern that an increase in resistanceand a decrease in capacity may be caused. From the above-describedviewpoint, the number of protrusions twice or more as thick as thecarbonaceous film is preferably two or less, more preferably one orless, and particularly preferably zero.

Meanwhile, a protrusion twice or more as thick as the carbonaceous filmrefers to a protrusion having a thickness that is larger than thethickness (average film thickness) of the carbonaceous film by twice ormore and having an area that is triple or less the thickness of theprotrusion.

The number of protrusions twice or more as thick as the carbonaceousfilm can be obtained by observing the electrode material using theelectron microscope under the above-described conditions and countingthe number of protrusions having a shape that satisfies theabove-described definition.

Method for Manufacturing Electrode Material

A method for manufacturing the electrode material of the presentembodiment has a first step of drying and granulating a slurry obtainedby mixing an ionic organic substance as a carbon source, one or moreselected from an electrode active material and an electrode activematerial precursor, and a solvent using a spray dryer and a second stepof thermally treating the granulated substance obtained in the firststep in a non-oxidative atmosphere at 600° C. or higher and 1,000° C. orlower.

First Step

The present step is a step of drying and granulating a slurry obtainedby mixing an ionic organic substance as a carbon source, one or moreselected from an electrode active material and an electrode activematerial precursor, and a solvent using a spray dryer.

As the ionic organic substance, the electrode active material, and/orthe electrode active material precursor, the ionic organic substance,the electrode active material, and the electrode active materialprecursor described in the section of “Electrode material” respectivelycan be used.

When the ionic organic substance is used as a carbon source, it ispossible to suppress the migration of the organic substance during thedrying and granulation of the slurry obtained by mixing the ionicorganic substance, one or more selected from the electrode activematerial and the electrode active material precursor, and a solvent anda thermal treatment described below, and it is possible to improve thecoating ratio of the carbonaceous film without the organic substance(carbide) being freed from the carbonaceous film. Therefore, poorlycarbonaceous organic substances (carbides) which do not contribute tocoating are not generated, the surfaces of the electrode active materialparticles are appropriately coated with the organic substance (carbide),and it is possible to form favorable carbonaceous films.

In addition, the electrode active material particles coming close to oneanother are suppressed through the charge repulsion and the stericbarrier of the ionic organic substance, the particle growth andsintering of the electrode active material due to high temperatures donot easily occur, and fine electrode active material particles coatedwith a favorable carbonaceous film can be obtained.

First, an ionic organic substance and one or more selected from anelectrode active material and an electrode active material precursor aredissolved or dispersed in a solvent, thereby preparing a mixture. Amethod for dissolving or dispersing the ionic organic substance and oneor more selected from the electrode active material and the electrodeactive material precursor in the solvent is not particularly limited,and it is possible to use, for example, a dispersion device such as aplanetary ball mill, an oscillation ball mill, a bead mill, a paintshaker, or an attritor.

Examples of the solvent include water; alcohols such as methanol,ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol,pentanol, hexanol, octanol, and diacetone alcohol; esters such as ethylacetate, butyl acetate, ethyl lactate, propylene glycol monomethyl etheracetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone;ethers such as diethyl ether, ethylene glycol monomethyl ether (methylcellosolve), ethylene glycol monoethyl ether (ethyl cellosolve),ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycolmonomethyl ether, and diethylene glycol monoethyl ether; ketones such asacetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK),acetylacetone, and cyclohexanone; amides such as dimethyl formamide,N,N-dimethylacetoacetamide, and N-methylpyrrolidone; glycols such asethylene glycol, diethylene glycol, and propylene glycol; and the like.These solvents may be used singly, or two or more solvents may be usedin mixture. Among these solvents, a preferred solvent is water.

Meanwhile, a dispersant may be added as necessary.

The blending ratio between the ionic organic substance and one or moreselected from the electrode active material and the electrode activematerial precursor is preferably 0.5 parts by mass or more and 10 partsby mass or less in terms of the amount of a carbonaceous substanceobtained from the ionic organic substance with respect to 100 parts bymass of an active material that is obtained from one or more selectedfrom the electrode active material and the electrode active materialprecursor. The actual blending amount varies depending on thecarbonization amount (the kind or carbonization conditions of the carbonsource) by means of heating carbonization and is approximately 1 part bymass to 8 parts by mass.

In addition, when the ionic organic substance and one or more selectedfrom the electrode active material and the electrode active materialprecursor are dissolved and dispersed in a solvent, it is preferable todisperse one or more selected from the electrode active material and theelectrode active material precursor in the solvent and then add and stirthe ionic organic substance.

Next, the obtained slurry is dried and granulated using a spray dryer,whereby a granulated substance can be obtained.

Second Step

The present step is a step of thermally treating the granulatedsubstance obtained in the first step in a non-oxidative atmosphere at600° C. or higher and 1,000° C. or lower.

The non-oxidative atmosphere is preferably an inert atmosphere ofnitrogen (N₂), argon (Ar), or the like, and, in a case in which it isnecessary to further suppress oxidation, a reducing atmosphere includingapproximately several percentages by volume of a reducing gas such ashydrogen (H₂) is preferred. In addition, for the purpose of removingorganic components evaporated in the non-oxidative atmosphere during thethermal treatment, a susceptible or burnable gas such as oxygen (O₂) maybe introduced into the inert atmosphere.

The thermal treatment is carried out at a temperature in a range of 600°C. or higher and 1,000° C. or lower, preferably 700° C. or higher and900° C. or lower, more preferably 720° C. or higher and 900° C. orlower, and still more preferably 725° C. or higher and 850° C. or lowerfor 1 to 24 hours, preferably 1 to 12 hours, more preferably 1 to 8hours, and still more preferably 2 to 4 hours.

When the thermal treatment temperature is lower than 600° C., thecarbonization of the ionic organic substance becomes insufficient, andthere is a concern that it may be impossible to increase electronconductivity, and, when the thermal treatment temperature is higher than1,000° C., there is a concern that the electrode active materialparticles may be decomposed or the suppression of particle growth may beimpossible.

According to the manufacturing method of the present embodiment, theionic organic substance having an excellent adsorption capability toparticle surfaces is used as the precursor of the carbonaceous film, andthus the coatability is enhanced. In addition, it is possible tosuppress the electrode active material particles coming close to oneanother through charge repulsion and a steric barrier, and thus it ispossible to carbonize the ionic organic substance at a high temperature,and it is possible to easily obtain fine and highly reactive electrodematerials coated with a carbonaceous substance having higher electronconductivity without excessively including carbon and, additionally, torealize excellent carbonaceous films capable of decreasing the poorlycarbonaceous carbide and suppressing an increase in resistance by thedecomposition of the poorly carbonaceous carbide during cycle chargingand discharging. Furthermore, it is possible to obtain lithium ionbatteries having excellent cycle characteristics by producing granularelectrode materials which can be easily turned into pastes and haveexcellent coatability.

The manufacturing method of the present embodiment is applicableregardless of the kind of the electrode active material and isparticularly effective as a method for manufacturing olivine-typephosphate-based electrode materials having low electron conductivity dueto the low costs and low environmental loads.

Electrode

An electrode of the present embodiment is formed of the electrodematerial of the present embodiment.

In order to produce the electrode of the present embodiment, theelectrode material, a binder made of a binder resin, and a solvent aremixed together, thereby preparing paint for forming an electrode orpaste for forming an electrode. At this time, a conductive auxiliaryagent such as carbon black, acetylene black, graphite, ketjen black,natural graphite, or artificial graphite may be added thereto asnecessary.

As the binder, that is, the binder resin, for example, apolytetrafluoroethylene (PTFE) resin, a polyvinylidene fluoride (PVdF)resin, fluorine rubber, or the like is preferably used.

The blending ratio between the electrode material and the binder resinis not particularly limited; however, for example, the content of thebinder resin is set to 1 part by mass or more and 30 parts by mass orless and preferably set to 3 parts by mass or more and 20 parts by massor less with respect to 100 parts by mass of the electrode material.

The solvent that is used for the paint for forming an electrode or thepaste for forming an electrode may be appropriately selected inaccordance with the properties of the binder resin.

Examples thereof include water, alcohols such as methanol, ethanol,1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol,hexanol, octanol, and diacetone alcohol, esters such as ethyl acetate,butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate,propylene glycol monoethyl ether acetate, and γ-butyrolactone, etherssuch as diethyl ether, ethylene glycol monomethyl ether (methylcellosolve), ethylene glycol monoethyl ether (ethyl cellosolve),ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycolmonomethyl ether, and diethylene glycol monoethyl ether, ketones such asacetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK),acetylacetone, and cyclohexanone, amides such as dimethyl formamide,N,N-dimethylacetoacetamide, and N-methylpyrrolidone (NMP), glycols suchas ethylene glycol, diethylene glycol, and propylene glycol, and thelike. These solvents may be used singly, or a mixture of two or moresolvents may be used.

Next, the paint for forming an electrode or the paste for forming anelectrode is applied onto one surface of a metallic foil and then isdried, thereby obtaining a metallic foil having a coated film made of amixture of the electrode material and the binder resin formed on onesurface.

Next, the coated film is pressed under pressure and is dried, therebyproducing a current collector (electrode) having an electrode materiallayer on one surface of the metallic foil.

In the above-described manner, it is possible to produce electrodescapable of the obtainment of lithium ion batteries in which the directcurrent resistance is decreased, the discharge capacity and the inputand output characteristics are excellent, an increase in resistanceduring cycle charging and discharging is suppressed, and the cyclecharacteristics are also excellent.

Lithium Ion Battery

A lithium ion battery of the present embodiment includes a cathode madeof the electrode of the present embodiment. Therefore, the lithium ionbattery of the present embodiment decreases the direct currentresistance, is excellent in terms of the discharge capacity and theinput and output characteristics, suppresses an increase in resistanceduring cycle charging and discharging, and is also excellent in terms ofthe cycle characteristics.

In the lithium ion battery of the present embodiment, an anode, anelectrolyte, a separator, and the like are not particularly limited. Forexample, as the anode, it is possible to use an anode material such asmetallic Li, a carbon material, a Li alloy, or Li₄Ti₅O₁₂. In addition,instead of the electrolyte and the separator, a solid electrolyte may beused.

EXAMPLES

Hereinafter, the present invention will be specifically described usingexamples and comparative examples. Meanwhile, the present invention isnot limited to forms described in the examples.

For example, in the present examples, acetylene black is used as aconductive auxiliary agent, but a carbon material such as carbon black,graphite, Ketjen black, natural graphite, or artificial graphite mayalso be used. In addition, batteries in which natural graphite is usedas a counter electrode will be evaluated, but it is needless to say thatother carbon materials such as artificial graphite and coke, metallicanodes such as metallic Li or Li alloys, and oxide-based anode materialssuch as Li₄Ti₅O₁₂ may also be used. In addition, as a non-aqueouselectrolyte (a solution of a non-aqueous electrolyte), an electrolytewhich includes 1 mol/L of LiPF₆ and is produced by mixing ethylenecarbonate and ethyl methyl carbonate 3:7 in terms of % by volume isused, but an electrolyte in which LiBF₄ or LiClO₄ is used instead ofLiPF₆ and propylene carbonate or diethyl carbonate is used instead ofethylene carbonate may be used. In addition, instead of the electrolyteand the separator, a solid electrolyte may be used.

Manufacturing Example 1: Manufacturing of Electrode Active Material(LiFePO₄)

LiFePO₄ was hydrothermally synthesized in the following manner.

LiOH as a Li source, NH₄H₂PO₄ as a P source (B source), and FeSO₄.7H₂Oas a Fe source (A source) were used and were mixed into pure water sothat the substance amount ratio (Li:Fe:P) therebetween reached 3:1:1,thereby preparing a homogeneous slurry-form mixture (200 mL).

Next, this mixture was put into a pressure-resistant airtight containerhaving a capacity of 500 mL and was hydrothermally synthesized at 170°C. for 12 hours. After this reaction, the mixture was cooled to roomtemperature (25° C.), thereby obtaining a cake-form reaction productwhich was precipitated in the container. This precipitate wassufficiently cleaned a plurality of times with distilled water, and thewater content ratio was maintained at 30% so as to prevent theprecipitate from being dried, thereby producing a cake-form substance. Aslight amount of this cake-form substance was sampled, powder obtainedby drying the cake-form substance in a vacuum at 70° C. for two hourswas measured using an X-ray diffractometer (product name: RINT2000,manufactured by Rigaku Corporation), and it was confirmed thatsingle-phase LiFePO₄ was formed.

Manufacturing Example 2: Manufacturing of Electrode Active Material(LiMnPO₄)

LiMnPO₄ was synthesized in the same manner as in Manufacturing Example 1except for the fact that MnSO₄.H₂O was used instead of FeSO₄.7H₂O as theFe source and the reaction was a hydrothermal synthesis at 150° C. for12 hours.

Manufacturing Example 3: Manufacturing of Electrode Active Material(Li[Fe_(0.25)Mn_(0.75)]PO₄)

Li[Fe_(0.25)Mn_(0.75)]PO₄ was synthesized in the same manner as inManufacturing Example 2 except for the fact that a mixture of FeSO₄.7H₂Oand MnSO₄.H₂O (at a substance amount ratio of 25:75) was used as the Fesource.

Example 1

LiFePO₄ (electrode active material) (20 g) obtained in ManufacturingExample 1 and ammonium dodecylbenzenesulfonate (1 g) as a carbon sourcewere mixed into water and crushed and mixed using a ball mill togetherwith zirconia balls (150 g) having 5 mmϕ, thereby obtaining a slurry(mixture).

Next, the obtained slurry was dried and granulated using a two-fluidnozzle-type spray dryer. After that, the obtained granulated substancewas thermally treated at a temperature of 750° C. for four hours,thereby obtaining an electrode material made of a carbonaceous-coatedelectrode active material.

Example 2

LiMnPO₄ (electrode active material) (19 g) obtained in ManufacturingExample 2, a mixed solution of lithium carbonate, iron (II) acetate, andphosphoric acid (Li:Fe:P=1:1:1) equivalent to 1 g of as LiFePO₄ acarbonization catalyst, and ammonium dodecylbenzenesulfonate (2.5 g) asa carbon source were mixed into water and crushed and mixed using a ballmill together with zirconia balls (150 g) having 5 mmϕ, therebyobtaining a slurry (mixture).

Next, the obtained slurry was dried and granulated using a two-fluidnozzle-type spray dryer. After that, the obtained granulated substancewas thermally treated at a temperature of 750° C. for four hours,thereby obtaining an electrode material made of a carbonaceous-coatedelectrode active material.

Example 3

Li[Fe_(0.25)Mn_(0.75)]PO₄ (electrode active material) (20 g) obtained inManufacturing Example 3 and carboxylic acid-modified polyvinyl alcohol(manufactured by Kuraray Co., Ltd., trade name: KL-318, 1.2 g) as acarbon source were mixed into water and neutralized using ammonia waterso that the neutralization ratio of a carboxyl group of the carboxylicacid-modified polyvinyl alcohol reached 100 mol %. After that, thecomponents were crushed and mixed using a ball mill together withzirconia balls (150 g) having 5 mmϕ, thereby obtaining a slurry(mixture).

Next, the obtained slurry was dried and granulated using a two-fluidnozzle-type spray dryer. After that, the obtained granulated substancewas thermally treated at a temperature of 750° C. for four hours,thereby obtaining an electrode material made of a carbonaceous-coatedelectrode active material.

Example 4

An electrode material made of a carbonaceous-coated electrode activematerial was obtained in the same manner as in Example 3 except for thefact that the carboxyl group was neutralized so that the neutralizationratio of the carboxyl group of the carboxylic acid-modified polyvinylalcohol (manufactured by Kuraray Co., Ltd., trade name: KL-318) reached50 mol %.

Example 5

Li[Fe_(0.25)Mn_(0.75)]PO₄ (electrode active material) (20 g) obtained inManufacturing Example 3 and an ammonium carboxylate-based surfactant(manufactured by Toagosei Co., Ltd., trade name: ARON A-6114, 1.2 g) asa carbon source were mixed into water. After that, the components werecrushed and mixed using a ball mill together with zirconia balls (150 g)having 5 mmϕ, thereby obtaining a slurry (mixture).

Next, the obtained slurry was dried and granulated using a two-fluidnozzle-type spray dryer. After that, the obtained granulated substancewas thermally treated at a temperature of 780° C. for four hours,thereby obtaining an electrode material made of a carbonaceous-coatedelectrode active material.

Comparative Example 1

An electrode material made of a carbonaceous-coated electrode activematerial of Comparative Example 1 was obtained in the same manner as inExample 2 except for the fact that sucrose (2.5 g) was used instead ofthe ammonium dodecylbenzenesulfonate (2.5 g) as a carbon source.

Comparative Example 2

An electrode material made of a carbonaceous-coated electrode activematerial of Comparative Example 2 was obtained in the same manner as inExample 3 except for the fact that the carboxylic acid-modifiedpolyvinyl alcohol (manufactured by Kuraray Co., Ltd., trade name:KL-318) was used without being neutralized.

Comparative Example 3

An electrode material made of a carbonaceous-coated electrode activematerial of Comparative Example 3 was obtained in the same manner as inExample 3 except for the fact that native polyvinyl alcohol was usedinstead of the carboxylic acid-modified polyvinyl alcohol (manufacturedby Kuraray Co., Ltd., trade name: KL-318).

Production of Lithium Ion Batteries

The electrode material obtained in each of the examples and thecomparative examples, acetylene black (AB) as a conductive auxiliaryagent, and a polyvinylidene fluoride (PVdF) resin as a binder were mixedinto N-methylpyrrolidone (NMP) so that the weight ratio (the electrodematerial:AB:PVdF) therebetween reached 90:5:5, thereby producing cathodematerial paste. The obtained paste was applied and dried on a 30μm-thick aluminum foil and was pressed so as to obtain a predetermineddensity, thereby producing an electrode plate.

A plate-like specimen having a coated surface with an area of 3×3 cm²and an allowance for a tab around the coated surface was obtained fromthe obtained the electrode plate by means of punching, and the tap waswelded, thereby producing a test electrode.

Meanwhile, as a counter electrode, similarly, a coated electrodeobtained by applying natural graphite was used. As a separator, a porouspolypropylene film was employed. In addition, a lithiumhexafluorophosphate (LiPF₆) solution (1 mol/L) was used as a non-aqueouselectrolyte (a solution of a non-aqueous electrolyte). Meanwhile, as asolvent that was used in the LiPF₆ solution, a solvent obtained bymixing ethylene carbonate and ethyl methyl carbonate 3:7 in terms of %by volume and adding vinylene carbonate (2%) thereto as an additive wasused.

In addition, laminate-type cells were produced using the test electrode,the counter electrode, and the non-aqueous electrolyte produced in theabove-described manner and were used as batteries of the examples andthe comparative examples.

Evaluation of Electrode Materials

For the electrode materials obtained in the examples and the comparativeexamples and the components included in the electrode materials,physical properties were evaluated. The evaluation methods are asdescribed below. Meanwhile, the results are shown in Table 1.

1. Amount of Carbon in Electrode Material

The amount (% by mass) of carbon in the electrode material was measuredusing a carbon analyzer (manufactured by Horiba Ltd., carbon/sulfurcombustion analyzer EMIA-810W).

2. Specific Surface Area of Electrode Material

The specific surface area of the electrode material was measured using aspecific surface area meter (for example, manufactured by Mountech Co.,Ltd., trade name: Macsorb HM model-1208) and the BET 1 point method bymeans of nitrogen (N₂) adsorption.

3. Crystallite Diameter of Electrode Active Material

The crystallite diameter of the electrode active material was calculatedfrom the Scherrer's equation using the full width at half maximum of thediffraction peak and the diffraction angle (2θ) of the (020) plane in apowder X-ray diffraction pattern measured using an X-ray diffractometer(product name: RINT2000, manufactured by Rigaku Corporation).

4. Evaluation of Carbonaceous Films in Electrode Materials

The electrode material was captured using a transmission electronmicroscope (TEM; manufactured by Hitachi High-Technologies Corporation,product No.: HF2000) at an accelerated voltage of 200 kV and amagnification of 100,000 times. In the obtained image of a crosssection, ten random 180 nm×180 nm views were observed, the number ofirregular carbonaceous lumps freed from the carbonaceous film wascounted, and this number was regarded as the number of free carbonaggregates.

In addition, similarly, in the obtained image of the cross section, tenrandom 180 nm×180 nm views were observed, the thickness of thecarbonaceous film was measured at 100 places, and the average valuethereof was regarded as the thickness of the carbonaceous film.Furthermore, in the obtained image of the cross section, ten random 180nm×180 nm views were observed, the number of protrusions having athickness (T) that was larger than the thickness of the carbonaceousfilm by twice or more and having an area that was triple or less aslarge as T was counted, and this number was regarded as the number ofprotrusions twice or more as thick as the carbonaceous film.

Meanwhile, the image of the cross section obtained by capturing theelectrode material of Example 3 using a transmission electron microscope(TEM) is illustrated in FIG. 1, and the image of the cross sectionobtained by capturing the electrode material of Comparative Example 2using a transmission electron microscope (TEM) is illustrated in FIG. 2.

Evaluation of electrodes and lithium ion batteries Discharge capacitiesand direct current resistances (DCR) of charging and discharging weremeasured using the lithium ion batteries obtained in the examples andthe comparative examples. The results are shown in Table 1.

1. Discharge Capacity

Discharge capacities were measured at an ambient temperature of 30° C.by means of constant-electric current charging and discharging with acharge electric current set to 1 C, a discharge electric current set to5 C, and a cut-off voltage set to 2.5 to 4.1 V (vs natural graphite) forthe lithium ion battery of Examples 1 and to 2.5 to 4.2 V (vs naturalgraphite) for the lithium ion batteries of Examples 2 to 5 andComparative Examples 1 to 3.

2. Direct Current Resistance (DCR) of Charging and Discharging

The lithium ion batteries were charged with an electric current of 0.1 Cat an ambient temperature of 25° C. for five hours, and the depths ofcharge were adjusted (state of charge (SOC) 50%). On the batteriesadjusted to SOC 50%, “1 C charging for 10 seconds→10-minute rest→1 Cdischarging for 10 seconds→10-minute rest” as a first cycle, “3 Ccharging for 10 seconds→10-minute rest→3 C discharging for 10seconds→10-minute rest” as a second cycle, “5 C charging for 10seconds→10-minute rest→5 C discharging for 10 seconds→10-minute rest” asa third cycle, and “10 C charging for 10 seconds→10-minute rest→10 Cdischarging for 10 seconds→10-minute rest” as a fourth cycle weresequentially carried out. Voltages 10 seconds after the respectivecharging and discharging during the cycles were measured. Individualelectric current values were plotted along the horizontal axis, and thevoltages after 10 seconds were plotted along the vertical axis, therebydrawing approximate straight lines. The slopes of the approximatestraight lines were respectively considered as direct currentresistances during charging (charging DCR) and direct currentresistances during discharging (discharging DCR).

Similarly, DCR after the cycle test was measured at an ambienttemperature of 25° C.

3. Charge and Discharge Cycle Test

After the measurement of the discharge capacity and DCR, 300 cycles ofcharging and discharging were carried out at an ambient temperature of45° C. with charge and discharge electric currents set to 2 C. Thecut-off voltage was set to the same voltage as in the measurement of thedischarge capacity of 1.

From the discharge capacity after 300 cycles and the discharge capacityat the second cycle, the cycle retention rate was calculated using thefollowing equation (i)

Cycle retention rate (%)=(discharge capacity at 300^(th) cycle/dischargecapacity at 2^(nd) cycle)×100  (i)

TABLE 1 Amount of Specific Crystallite carbon in surface area ofdiameter of Thickness of Number of electrode electrode electrodecarbonaceous free carbon Electrode active material material activematerial film aggregates material (% by mass) (m²/g) (nm) (nm) (atoms)Example 1 LiFePO₄ 1.2 16 75 2.1 0 Example 2 LiMnPO₄ 2.0 26 50 4.2 1Example 3 Li[Fe_(0.25)Mn_(0.75)]PO₄ 1.9 26 51 2.2 0 Example 4Li[Fe_(0.25)Mn_(0.75)]PO₄ 1.8 23 52 2.4 2 Example 5Li[Fe_(0.25)Mn_(0.75)]PO₄ 1.9 27 50 2.3 0 Comparative LiMnPO₄ 1.7 16 864 7 Example 1 Comparative Li[Fe_(0.25)Mn_(0.75)]PO₄ 1.7 14 87 2.2 4Example 2 Comparative Li[Fe_(0.25)Mn_(0.75)]PO₄ 1.7 14 87 2.3 4 Example3 Number of protrusion twice or more as thick as 5 C Cycle After cycletest carbonaceous discharge Charging Discharging retention Dischargingfilm capacity DCR DCR rate Charging DCR DCR (protrusions) (mAh/g) (Ω)(Ω) (%) (Ω) (Ω) Example 1 0 145 5.2 4.1 92 4.9 4.5 Example 2 0 126 10.47.2 82 9.4 8.6 Example 3 0 130 9.8 6.6 86 8.6 7.3 Example 4 1 130 9.86.7 84 9.4 8.7 Example 5 0 131 9.7 6.5 87 8.5 7.2 Comparative 5 109 1610 59 32.0 25.0 Example 1 Comparative 3 110 13 8.5 66 17.9 16.2 Example2 Comparative 4 111 12.5 8.4 66 17.6 16.0 Example 3

SUMMARY OF RESULTS

In the examples, as the precursor of the carbonaceous film, the ionicorganic substances which had an excellent adsorption capability toparticle surfaces and was capable of suppressing the electrode activematerial particles coming close to one another through charge repulsionand a steric barrier were used, and thus the electrode materials made offine particles in which the particle growth and sintering of theelectrode active material due to high temperatures did not easily occur,fine electrode active material particles coated with a favorablecarbonaceous film were obtained, furthermore, the electrode activematerial particles coming close to one another was suppressed even whenspherical granular bodies were obtained by spraying and drying, andparticle growth did not occur were obtained.

In addition, it is found from the image of the cross section obtained bycapturing the electrode material of Example 3 using a transmissionelectron microscope (TEM) (at a magnification of 100,000 times)illustrated in FIG. 1 that, in the electrode material, the organicsubstance (carbide) was not freed, and no protrusions twice or more asthick as the carbonaceous film were confirmed. As such, in the electrodematerials of the examples, the organic substances (carbides) were notfreed, and the coating ratios improved, and thus poorly carbonaceousorganic substances (carbides) which did not contribute to coating werenot generated, and carbonaceous organic substances being decomposed andincreased in resistance by charging and discharging were suppressed.

It could be confirmed that, due to the above-described effects, theelectron conductivity of the carbonaceous films sufficiently improved,the discharge capacity increased, and the cycle characteristics alsoimproved.

On the other hand, in the electrode materials of the comparativeexamples, the growth of particles occurred in association with thethermal treatment, the organic substances (carbides) were freed evenduring the obtainment of spherical granular bodies by spraying anddrying, and poorly carbonaceous organic substances (carbides) which didnot contribute to electron conduction remained and were decomposed inassociation with charging and discharging. As a result, not only adecrease in the discharge capacity but also the deterioration of thecycle characteristics were observed.

Meanwhile, FIG. 2 is an image of a cross section obtained by capturingthe electrode material of Comparative Example 2 using a transmissionelectron microscope (TEM) (at a magnification of 100,000 times), but itis confirmed that, in the electrode material, the organic substance(carbide) was freed and protrusions twice or more as thick as thecarbonaceous film were confirmed.

The electrode material of the present invention is useful for cathodesin lithium ion batteries.

What is claimed is:
 1. An electrode material comprising: acarbonaceous-coated electrode active material having primary particlesof an electrode active material, secondary particles that are aggregatesof the primary particles, and a carbonaceous film that coats the primaryparticles of the electrode active material and the secondary particlesthat are the aggregates of the primary particles, wherein, in theelectrode material, when ten random 180 nm×180 nm views are observedusing an electron microscope at a magnification of 100,000 times, thenumber of free carbon aggregates is three or less, and the number ofprotrusions twice or more as thick as the carbonaceous film is three orless.
 2. The electrode material according to claim 1, wherein theelectrode active material is an electrode active material represented byGeneral Formula (1),Li_(a)A_(x)M_(y)BO_(z)  (1) (in the formula, A represents at least oneelement selected from the group consisting of Mn, Fe, Co, and Ni, Mrepresents at least one element selected from the group consisting ofNa, K, Mg, Ca, Al, Ga, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, and rare earthelements, B represents at least one element selected from the groupconsisting of B, P, Si, and S, 0≤a<4, 0<x<1.5, 0≤y<1, and 0<z≤4).
 3. Theelectrode material according to claim 2, wherein the electrode activematerial represented by General Formula (1) is an electrode activematerial represented by General Formula (2),Li_(a)A_(x)M_(y)PO₄  (2) (in the formula, A, M, a, x, and y are asdescribed above).
 4. A method for manufacturing the electrode materialaccording to claim 1, comprising: a first step of drying and granulatinga slurry obtained by mixing an ionic organic substance as a carbonsource, one or more selected from an electrode active material and anelectrode active material precursor, and a solvent using a spray dryer;and a second step of thermally treating the granulated substanceobtained in the first step in a non-oxidative atmosphere at 600° C. orhigher and 1,000° C. or lower.
 5. An electrode formed of the electrodematerial according to claim
 1. 6. A lithium ion battery comprising: acathode made of the electrode according to claim 5.